A NOTE ON AREALLY-AVERAGED EVAPORATION AND THE VALUE OF THE EFFECTIVE SURFACE CONDUCTANCE

The sensitivity of regional evaporation to spatial variability in surface conductance is investigated for vegetation with a fixed aerodynamic resistance. The effective surface conductance is shown to be a function only of the aerodynamic conductance and the probability density function, which describes the variation in g(s). Assuming a gamma probability distribution suggests fractional differences in evaporation and surface conductance which are typically less than 10% on average. Areal evaporation from dry regions is more sensitive to the spatial variability than evaporation from wet regions.